The use of various types of UV absorbers in the stabilization of polymers is well known. These stabilizers function by absorbing incident UV radiation and dispersing the absorbed energy in a nondestructive manner. Concentration of the UV absorber near the surface of the polymer to prevent penetration of UV light is, therefore, considered to be more efficient and economical than dispersion of the UV absorber throughout the bulk of the polymer.
The protection of aromatic polycarbonates from UV light is a particular problem as the polymer itself absorbs significant amounts of high energy light and is known to become quickly yellow and hazy. Aside from their strength and toughness, polycarbonates derive much of their value from their excellent optical properties, i.e. low color and high clarity. Therefore, protection against UV radiation at the surface of a polymer like polycarbonate, where most of the light would be absorbed, becomes vital.
It is of course obvious that any color imparted to the polymer composition by the stabilizer itself must be as little as possible.
A number of methods have been developed to concentrate UV absorbers near or at the surface of polymeric materials. These include surface impregnation (see U.S. Pat. Nos. 3,309,220, 3,043,709, 4,861,664 and 4,937,026) and coating a plastic article with solutions containing thermoplastic resins and UV absorbers (see U.S. Pat. Nos. 4,668,588 and 4,353,965). Both techniques suffer from significant drawbacks including requiring additional processing steps(i.e. applying, drying or curing), and encounter the difficulties associated with the handling of large processed articles. An additional drawback, particularly relevant to polycarbonate sheet production, is the detrimental effect such post addition treatment would have on the surface of the polymeric substrate.
Coextrusion is a well recognized method of producing laminated thermoplastic materials by simultaneously extruding various numbers of layers which form a single composite material. U.S. Pat. No. 4,540,623 describes coextruded materials of at least forty layers. Other methods produce as few as two or three different layers.
The resins employed in each layer may be the same, different e.g. polypropylene on polyacrylate on polycarbonate or be different types of similar resin such as two different aromatic polycarbonates.
Likewise, the types and amounts of additives employed in each layer may vary significantly. Typical additives include dyes, pigments, antioxidants, processing and UV stabilizers and glass fillers.
The use of coextrusion to produce a photo-stable article by incorporating a light absorbing layer at or near the surface of the laminate has received attention. Of particular importance is the protection of a polycarbonate core layer via such a process.
In EP 110,221, a polycarbonate core layer is protected with a thin polycarbonate layer which contains a high concentration of UV absorber. However, the UV absorbers used are volatile, especially under extrusion conditions, and loss and migration from the polymer composition may occur.
To prevent additive exudation, a third capping or barrier layer is incorporated which contains little or no volatile additive. In U.S. Pat. No. 4,707,393 a permanent layer of poly(methyl methacrylate) (PMMA) or PMMA composite is coextruded on top of the two polycarbonate layers. EP 345,884 describes the use of a removable barrier layer.
Recently, U.S. Pat. No. 5,108,835 teaches that the exudation problem of EP 110,221 can be overcome by incorporating certain low volatility bisbenzotriazole UV absorbers into the UV absorbing layer. These non-migrating UV absorbers make the capping layer of U.S. Pat. No. 4,707,393 or EP 345,884 unnecessary. This process claims the advantages of simplified production and perhaps more importantly, improved UV and weather stabilization of the thermoplastic moldings produced.
Canadian Patent Appl. 2,002,177 discloses a special polymethacrylate coating for aromatic polycarbonate which may be applied via coextrusion. This coating could contain a wide variety of UV absorbers for photostabilization of the polymers. However, the focus of this patent is to provide polycarbonate compatible polymethacrylate and provides no guidance toward the selection of UV absorber. The specific class of bisbenzophenones of the instant invention is not discussed.
Many of the UV absorbers corresponding to the formula of the present invention are known. U.S. Pat. Nos. 3,580,927 and 3,666,713; Czechoslovak Patent Appl. 135,115; Japanese Patent Applns. Sho 47-48888 and Sho 44-26456; and British Patent 1,396,240 describe the preparation and general use as UV stabilizers of a variety of compounds where Z in the formula of the instant bisbenzophenones is hydrogen. Instant compounds where Z is halogen, alkoxy or alkyl are described in Japanese Patent Appl. Hei 1-245046 and alluded to in Japanese Patent Appl. Sho 45-12852.
Other, more specific uses for some of these bisbenzophenone compounds are described in Japanese Patent Applns. Sho 44-3962; Sho 57-12643; Sho 56-24418; Sho 52-93462 and Hei 1-245046. These uses include stabilizing polypropylene, but deal mainly with specific polyester applications.
Nowhere in the existing prior an is the use of the instant bisbenzophenones in a UV absorbing layer of a laminated article contemplated nor is any coextrusion process involving said compounds disclosed.
In only one case, Japanese Sho 57-12643, is any type of laminated article discussed. However in this case the core layer is stabilized with a xylylene bridged bisbenzophenone and the protective coating layer is a non-absorbing water resistant copolyester-ether. A vague reference is made within this document to the core layer being a "composite film". However, the disclosure states and shows that the core layer must be a biaxially stretched UV stabilized film which is further coated. Such a coated film clearly cannot be produced via coextrusion.
Further, it was not obvious to apply these bisbenzophenone stabilizers to UV absorbing surface layers via coextrusion, or any other method. None of the prior art references foresaw the advantages that the use of these instant compounds provides in terms of better weathering and color development over the UV absorbers previously described. Specifically and most signficant is the improved weathering performance obtained by the use of these bisbenzophenone stabilizers over the bisbenzotriazoles described in U.S. Pat. No. 5,108,835 which is the current state of the art process.
The present invention offers an improvement over the existing prior art in that it not only provides the same simplification in production of photostabilized double walled thermoplastics found in U.S. Pat. No. 5,108,835, but also provides a laminated thermoplastic article with improved weathering resistance and lower color development than is currently available with the use of other classes of UV absorbers such as the bisbenzotriazoles.
This improvement in color and performance is particularly surprising since the bisbenzophenones of the instant invention and the bisbenzotriazoles of U.S. Pat. No. 5,108,835 demonstrate very similar color and weathering performance when incorporated via conventional bulk stabilization techniques into 125 mil (3.2 mm) thick plaques.